1. Field of the Invention
This invention relates to a device for producing blown films out of a thermoplastic plastic.
2. Discussion of Related Art
There are a wide variety of known devices of the above-mentioned type. The functional principle is based on the fact that the melted mass of thermoplastic plastic manufactured by at least one extruder is extruded from an annular exit opening of a die and blown into a tube, which is then folded by a flattening device, conveyed between squeezing rollers, and then supplied to a winding device.
In the simplest case, the flattening device is of two flattening elements, which are arranged in diametrically opposite regions of the tubular film and are adjustable at an acute angle in relation to each other, similar to an inverted V or an A, and whose distance from each other, when viewed in the exit direction of the tubular film, continuously decreases. The flattening elements have a number of guides such as strips, rollers, or brushes, which act on a circumference of the tubular film so that the tubular film, which is initially cylindrical, is gradually flattened, passing through an oval shape with continuously changing diameters, into the form of a double-layered film that is folded at its longitudinal edges.
If the oval cross-sectional shape of the gradually flattened tubular film, which shape takes form starting from the initially cylindrical tubular film, is viewed in the flattening device, then the tubular film, in the region of the larger diameter of the oval cross section at which the folded longitudinal edges of the tubular film finally form, is largely left alone during the flattening because it is not in contact with guide and is not guided by the flattening elements of the flattening device. This can lead to problems with regard to product stability, freedom from wrinkles, and the like.
U.S. Pat. No. 4,170,624 discloses transforming an initially cylindrical tubular film into a rectangular cross section in a special flattening device and then flattening it into a double layer. The intent of this is to avoid mechanical stress on the tubular film during the flattening and resulting changes in the properties profile of the tubular film.
Because of the straight arrangement of the brushes, rollers, or strips used as the guide for the tubular film, which has up to now been the default arrangement in the prior art, it is not possible to laterally guide the tubular film at a plurality of points in an ascending fashion. On the contrary, one result of the existing form is that only an insufficient point-by-point contact is possible and thus, particularly in reversing movements of the flattening device and/or a withdrawal device for the tubular film, which is situated after the squeezing rollers in order to improve the transverse thickness profile, the film starts to form slight wrinkles that run all the way through the completed roll and reduce the quality. This situation is clearly in need of improvement.